BIOEROSION EXPERIMENTS AT LIZARD-ISLAND, GREAT-BARRIER-REEF

The rates at which dead coral substrates are modified by bioerosional processes were determined by exposing recently killed corals for up to four years in a variety of reef environments at Lizard Island (northern Great Barrier Reef). Grazers were the major eroding agents of these coral substrates and exhibited differences between sites that varied between sampling periods. Sub-tidal reef slopes and lagoon environments of water depths < 20 m were subjected to higher average rates of grazing erosion (0.30-1.96 kg/m2/y) than shallow depths less than 1 m (0.07-0.26 k g/m2/y). A deep site at 20 m experienced low average rates of grazing (0.08-0.29 kg/m2/y). Boring rates by worms (polychaetes and sipunculans), sponges and molluscs were relatively low and varied between sites, but increased with length of sampling period as larger borers succeeded the initial colonizing small polychaete worms. We hypothesize from these experiments that the extent of boring in reef substrates will be influenced by the interaction between the succession of the boring community and the rate at which the substrate is destroyed by grazing. We suggest that the level of grazing modifies the successional pattern of borers by removing the surface substrate and continually exposing bare substrate that can be colonized by early boring colonists. Thus, constant high levels of grazing may maintain the boring community at an early successional stage and prevent the development of a ''mature'' boring community. In order to establish large borer populations, reef substrates must be protected from extensive grazing bioerosion. This interaction of grazing and boring has important implications for the way dead coral is preserved in different reef environments.